Welcome back, Robin! Isn't it fun planning your cruise?
If your only two port stops are Nassau and Castaway Cay, and you're looking to spend money on an adventure in one port only, I would recommend a Port Adventure in Nassau. Castaway Cay is Disney's private island and has an abundance of free fun going on all day long. There certainly are amazing
Port Adventures on Castaway Cay as well, but with all of the fun character dance parties, family whale dig times, fun for kids at Scuttle's Cove (while Mom sneaks away to Serenity Bay for a little adult time), water fun at Pelican Plunge, and more, you will find yourself busy and entertained for no additional charge all day long!
There are some fabulous
Port Adventures in Nassau. My family and I spent such a great day at Blue Lagoon Island and, though we did splurge on the Dolphin Swim, we could have spent the whole day just enjoying the island beach, which is available as its own less pricey adventure.
Atlantis is something to see for sure. Even if you don't do the Aquaventure water park there, just the Beach Day is amazing and you'll really enjoy wandering this resort. My husband and I were just there a few months ago during our Disney Cruise port call and especially enjoyed the aquariums. We felt like we were really in the ruins of the Lost City!
We prefer the relaxed convenience of letting Disney plan the details when we book port adventures, but keep in mind that guests are also welcome to walk off the ship on their own in Nassau. We've done that a few times too, and had great times shopping the main streets and browsing the straw market. You might want to attend the Port Shopping Talk onboard the day prior to get some discounts and tips for the best shopping areas in Nassau.
Happy Planning!
-Marcy